The demand for automobiles with electric propulsion is increasing due to the diminishing oil supply and concerns on Carbon emission to the atmosphere. Promising solutions including pure electric, and hybrid electric or fuel cell based motor vehicles. Each solution requires a battery pack with significant energy content, typically from 9 KWh for a small hybrid electric car to 300 KWh for EV buses. When such vehicle is decommissioned either due to accident or damages beyond repair, there is still significant energy stored in the battery pack. Furthermore, thermal management system might have been impaired to the point that battery cells are no longer protected. For vehicles involved in an accident, vehicle impact may have caused damages to the battery housing. Some battery cells may also have been damaged. Battery cells are known to have very long delay time between initial impact damage to short circuit. Battery packs 3 weeks after impact study have been reported to have caused vehicle fires. It is desirable to have a battery discharge system to safely drain away the stored energy to ensure the safety of the decommissioned battery pack.
FIG. 2 shows a typical battery module 100, with 5 battery cells, 101 through 105 connected in series. A typical battery system may have 20 of such modules connected in series, or for a total of 100 battery cells connected in series. For battery cells with Lithium Iron Phosphate technology, each cell output voltage is about 3.2 Volts. The battery system would have a total of 320 volts between its positive and negative terminals. Battery cell may be constructed by a single 100 Amp-hour prismatic cell, or a string of 50 smaller cylindrical format cells, each having 2 Amp-hour capacity, connected in parallel. When fully charged, total energy contained in this battery pack is 32 KW-hour, enough to power a small electric vehicle for 100 kilometers. When such vehicle is involved in an accident, the high energy content may be ignited, causing a vehicle fire, or be shorted to the chassis, causing a shock hazard to passengers and rescue works, or go dormant, until weeks later when one or more damaged cells developed into short circuit, and the energy released in the form of smoke, fire, or shock hazard. It is desirable to drain the battery energy away after such an accident. Prior art solution calls for external load, such as resistor bank, be connected to the battery terminal in order to drain away the stored energy. However, if the battery electrical circuit is damaged, for example, an open circuit developed in between battery cell 102 and 103, then the entire circuit is open, and the external solution does not work. Another scenario is that the pack is fully charged, but one cell, 104, is near empty (or low state of charge) due to impact damage, or charge imbalance. When an external resistor bank is connected to the battery system, cell 104 resistance increased very quickly due to the cell chemistry at low state of charge, and prevent the energy from other cells be depleted to the resistor bank. It is desirable to find a safe solution that has high probability of successfully draining the energy from a damaged battery pack in a controlled manner. It is desirable to find an automatic solution that can quickly drain the energy from a damaged pack as quickly after the incident as possible.
It is desirable to automatically and safely draining away the stored energy immediately following an accident to minimize safety risk to passengers as well as rescue workers. It is desirable to lower the energy stored in a battery pack to provide operational safety for the person transporting the decommissioned battery pack and the person guarding the warehouse where the decommissioned battery pack is stored.